Why an engine-room hit is the most disabling

Which hit would be most devastating to a warship: the engine room, medical bay, or rum store?

The engine room, because machinery damage can immobilize the ship; medical care can be redistributed, while the rum-store option is treated as a joke.

Of the three suggested targets, the engine room is the one most likely to threaten the ship's ability to continue operating. Losing propulsion and essential machinery can leave a warship unable to manoeuvre or sustain its systems. A damaged medical bay would be traumatic, but other treatment arrangements can be improvised or casualties transferred. The rum-store answer is treated as morale humour, reinforcing that the machinery casualty is the serious operational loss.